Vehicles with electric propulsion or hybrid propulsion, too, need a heating system for controlling the temperature of the vehicle cabin when outside temperatures are low. The heating energy comes from electric heating devices that are fed with electricity from the electric system of the vehicle. These heating devices are made up from a stack of identical heating units through which the air to be heated flows. In the heating device, PTC heating elements are used which are arranged between current rails that extend opposite each other, which means electric resistance elements having a positive temperature coefficient, which are fed with supply current from a control electronics unit. Metallic heating rib structures are respectively coupled to each of the current rails by means of an electrically insulating but thermally conducting coupling layer. The air to be heated flows through the heating rib structures, which have an undulating shape. The electric insulation of the heating rib structure from the current rails is necessary because the voltages taken from a high voltage electric system of the vehicle have high levels that are dangerous to humans. Consequently, in the known electrical heating devices of this type, the heating rib structures are potential-free.
However, in order to control the heating output, switched supply currents are fed to the PTC heating elements by the control electronics unit. The switching operations generate steep pulse edges which constitute a high frequency alternating current component of the supply current. As a result of these high frequency alternating current components, all live parts become radiators which in total emit a substantial amount of electromagnetic interfering radiation. Moreover, high frequency interference components from the on-board network of other vehicle units are already present in the supply currents for the PTC heating elements and can also be radiated via the live parts of the heating device. This interfering radiation can jeopardise the safe functioning of other systems in the vehicle as well as significantly affect receiving devices such as for example a radio.
In principle it is possible to surround the radiation-relevant parts of the heating device with an electromagnetic shield in the form of a metallic grid. However, since the grid will necessarily be located within the flow path of the air to be heated, it causes flow losses that would have to be compensated for by increasing the blower power.